Rents for housing in San Francisco have been soaring for the past 18 months, causing understandable alarm in a city where 65 percent of residents are renters. The problem is caused by exceptionally strong demand for housing and a structurally weak supply, but can be fixed with a new attitude and a new housing policy.

Over the past year, the City and County of San Francisco boasted the second strongest labor market in the nation, adding 25,000 new jobs. Yet only 2,548 new housing units were permitted and even fewer were built. Just think: 25,000 new workers and their families have been knocking on San Francisco doors, but there are new units for less than 10 percent of them. It is not surprising that apartment prices get bid up.

But job growth does not need to boost rents. This is where our policymakers have failed. Due to one of the slowest and most cumbersome housing approval processes in the nation, San Francisco is structurally unable to add the housing its workforce requires.

Data from 2,649 municipalities indicate that San Francisco's housing policies are among the most restrictive in the United States. We limit new construction through stringent land-use restrictions, incredible delays and lawsuits challenging proposed developments. Some restrictions make sense: We don't want skyscrapers in the Sunset; nor should we tear down historical buildings.

But others are purely political and choke the supply of new units. Thanks to effective lobbying by local NIMBY groups and the radical Left - progressive supervisors, the Tenants Union and the Mission Anti-Displacement Coalition among others - it takes years, harsh political battles and seemingly endless appeals to approve a new housing project.

Everything is political in San Francisco, but housing most of all. While savvy developers have learned how to navigate the cumbersome process, the renters are the main losers of this predicament.

A common misconception is that new housing only helps incoming residents. People see new, high-priced condos on Valencia Street and think that the only beneficiaries are new tech workers. When 1,000 new condos or apartments are built on Valencia Street, however, there are 1,000 fewer families competing with existing residents for housing. Everyone benefits.

Other progressive cities show us the advantages of an alternative approach. Job growth and housing demand growth in the high-tech mecca of Seattle have been even stronger than San Francisco's. Yet, thanks to a more rational and less political entitlement process, Seattle has added thousands of new units in its urban core, mostly via smart infill developments near transit, resulting in more and cheaper housing. In general, economic studies confirm that cities with a responsive housing supply experience much smaller rent increases than cities with rigid housing supply.

A strong labor market is a good problem to have - hundreds of American cities would love to have such a problem - but high rents can have serious social consequences. It is irresponsible to try to limit housing demand, because that would require limiting employment growth. But San Francisco can follow the examples of other progressive cities, and increase housing supply by:

For this to happen, it is first necessary that San Francisco's political Left moderates its preconceived opposition to all new market-based housing. The main winners would be the city's most vulnerable group, San Francisco renters.

Enrico Moretti is professor of economics at UC Berkeley and author of the book "The New Geography of Jobs" (Houghton Mifflin, 2012).